The present invention is related to a method for dynamic focus control, and more particularly to a method that performs dynamic focusing of a coherent array imaging system.
Dynamic focusing provides high focusing quality over the entire depth of interest, such as U.S. Pat. No. 5,581,517 and U.S. Pat. No. 5,111,695. In an array imaging system, focusing is typically done by first delaying the backscattered signals based on the propagation path length difference. The delayed signals are then coherently summed across the array. This is also known as the delay-and-sum approach. Since the medical ultrasound imaging primarily works in the near field region, the focusing delay for a particular channel changes as a function of range. Thus, the delay value must be dynamically updated. In other words, the real-time interpolation is required to increase the effective data sampling rate for higher focusing accuracy. Considering the large number of system channels, implementation of dynamic focus control is complicated. The complexity becomes more significant when a fully sampled, two-dimensional array is used. In this case, the number of channels can be as large as several thousands. Thus, efficient dynamic focus control schemes must be developed to reduce the system complexity.
Imaging with two-dimensional arrays has gained broad interest in the past few years. Potential advantages of such a system include reduced slice thickness, improved correction of sound velocity inhomogeneities and real-time three-dimensional imaging. Despite of the potential benefits, two-dimensional arrays have not been widely used in medical ultrasound. Particularly for three-dimensional imaging with two-dimensional (i.e. lateral and elevational) electronic steering, fully sampled arrays are required. The implementation is not possible with current electronic technologies unless major simplification can be achieved without significant image quality degradation.
Dynamic focusing in ultrasonic array imaging involves extensive real-time computations and data communication. Particularly for real-time three-dimensional imaging using fully-sampled two-dimensional arrays, the implementation of dynamic focusing can be extremely complicated.
According to the prior art, the dynamic focusing using arrays was performed without grouping adjacent channels and with uniform delay quantization.
Therefore, conventional approaches control individual channels independently. As the total channel count increases, this becomes impractical. Uniform quantization of delay values also results in a waste in system resources.
It is therefore a primary objective of the present invention to provide a method for dynamic focus control which describes details of the focus control scheme and demonstrates its efficacy for three-dimensional imaging using two-dimensional arrays. By exploiting characteristics of the range dependent focusing term, complexity is significantly reduced and implementation of dynamic receive focusing becomes much more feasible. Note that although the algorithms are developed for two-dimensional arrays, the same principles can be easily adopted to systems using one-dimensional arrays.
It is another objective of the present invention to provide a method for dynamic focus control which can remove a time waste without sacrificing the focusing quality by grouping adjacent channels and/or non-uniform quantization.
It is further an objective of the present invention to provide a method for dynamic focus control which designs a delay controller for dynamic focusing using arrays.
It is still another object of the present invention to provide a method for dynamic focus control which greatly simplifies the delay control architecture by exploiting characteristics of focusing delay patterns. With the invention, the beam former can be implemented with a substantially low size and cost. Advantages of the invention become more significant when the system channel count is large (e.g., in fully sampled 2D arrays).
It is further a more object of the present invention to provide a method for dynamic focus control which divides the focusing term into a range independent term and a term inversely proportional to the range. Since the second term decreases as the range increases, approximation can be made to simplify the control architecture with the focusing error decreasing as the range increases.
According to the present invention, it is provided a method of dynamic focus control for an array imaging system having an array of channels, each of which has a delay value and a delay controller for performing a delay control. In the method of dynamic focus control is characterized in that the delay value of a particular said channel is divided into a range independent component and a range dependent component, wherein the range dependent compontent is inversely proportional to a range and has a corresponding "PHgr" value dependent on the location of said channel.
In accordance with the present invention, the array of channels is divided into a plurality of sub-arrays.
In accordance with the present invention, all the channels in a sub-array have a common initial delay parameter "PHgr"xe2x80x2.
In accordance with the present invention, determination of a sub-array geometry is critical in minimizing the focusing error.
Preferably, the array of channels is a one-dimensional array.
Preferably, the array of channels is a two-dimensional array.
Preferably, the two-dimensional array is divided into concentric rings.
Preferably, each of the ring is further divided into smaller segments in order to reduce potential approximation errors for off-center beams, and channels inside a specific segment use a specific common "PHgr"xe2x80x2 value and thus specific delay controller.
Preferably, the "PHgr"xe2x80x2 value is defined as the mean of all the respective "PHgr" values within the specific segment.
Preferably, the "PHgr" value is non-uniformly quantized.
Preferably, a number of delay change patterns are quantized via said non-uniform quantization.
Preferably, the delay controller is a multiplexer that is used to select an entry of said delay change pattern.
Preferably, the multiplexer only needs a portion of a delay table.
Preferably, focus quality is finely tuned in range by modifying the initial delay such that the delay error is zero at a reference range.